The reduction of NOx emissions from diesel vehicles is desired. One approach uses a urea Selective Catalytic Reduction (SCR) exhaust after-treatment system. In the urea-based SCR catalyst, a reductant, such as NH3 (formed from the injection of aqueous urea, for example), reacts selectively with NOx with the products being N2 and H20.
In one approach, a control system attempts to control an amount of reductant stored on the urea-SCR system, in order to provide sufficient reductant for NOx reaction, but while reducing reductant emitted. For example, a model may be used to predict the NH3 storage in the catalyst based on various operating parameters, which may include exhaust sensors. To reduce modeling errors, such as due to changes in the NH3 storage capability of the catalyst, ammonia sensor downstream of the catalyst may be used.
U.S. Pat. No. 6,240,722 describes an example approach where a sensor indicates the NH3 storage levels of the SCR-catalyst. Specifically, the fullness of a reagent on a catalyst is determined by measuring some physical property that changes with gas storage. For example, NH3 storage on an SCR catalyst can be determined by measuring the change in impedance of a portion of the catalyst due to changes in NH3 storage.
The inventors herein have recognized a disadvantage of such an approach. For example, depending on the operating conditions, the change in impedance may not be significant or detectable and thus accuracy may be degraded.
To address at least some of the above issues, a system or method may be used in which the catalyst material itself can be used, where the conductivity of a portion of the catalyst material may be correlated to a stored reductant level, such as an amount of stored NH3 on the catalyst portion. In one particular embodiment, a method may be used where the conductivity may be monitored over a temperature change sufficient to desorb (or oxidize) stored reductant. As such, the resulting change in conductivity upon the catalyst material heating may be correlated (e.g., via a measured current) to an amount of reductant loading. The temperature change may be induced repeatedly at a frequency to provide sufficient information for updated estimates of reductant storage and to provide adjustment to injected reductant. Further, various additional factors may be considered in correlating the conductivity to reductant loading, such as the storage duration, reductant concentration, and exhaust gas temperature. In another embodiment, a plurality of sensors may be used at multiple locations along a direction of exhaust gas flow (e.g., length) of the catalyst to provide more accurate control of reductant injection, as well as for catalyst diagnostics, etc. In still another embodiment, the sensed reductant storage information may be used to adjust reductant injection.
In one particular embodiment, a system may utilize electrodes which are embedded within an NH3-storing layer of an SCR catalyst, along with an embedded heater used to desorb ammonia stored in the catalyst in the vicinity of the heater at selected conditions. The system may also determine temperature of the catalyst material between the electrodes and in the vicinity of the heater. Specifically, the embedded heater allows a portion of the catalyst substrate to be heated to a higher temperature than surrounding or adjacent substrate, thus liberating the stored NH3 in the heated portion. The resulting change in electrical conductivity of the heated portion, upon heating, may then be correlated to an overall ammonia loading on the SCR catalyst. Thus, information about the stored NH3 levels prior to heating can be obtained and overall ammonia storage capacity of the catalyst can be inferred.
An advantage of such an approach is that by heating up a portion of the catalyst and desorbing some or all of the stored ammonia from that portion, a higher and more accurate signal may be generated.